Throughout the world, numerous offshore reserves of oil and gas have been discovered and developed. This trend of discovering, drilling, and developing offshore fields of oil and gas is expected to continue and accelerate as the demand for energy increases and fields on the land become depleted or otherwise economically unattractive.
After an offshore reserve has been discovered and tested, it is necessary to drill one or a number of production wells to remove the oil and/or gas. Typically, at any one offshore well site, a number of wells will be spaced closely together so as to facilitate drilling and collection of the commodity. The commodity, oil and/or gas, removed from the reserve by the wells requires at least some processing before loading onto a surface vessel such as a tanker. Accordingly, it has been known to moor a surface production vessel above the well site to receive and at least partially process the oil or gas from the well.
Mooring a surface vessel at sea encounters several drawbacks. One drawback is that a surface vessel is exposed to forces of waves, wind and surface currents, all of which are most pronounced at or near the ocean's surface. Adverse weather such as hurricanes, typhoons, or the like, increase the surface forces. To moor the surface vessel, therefore, requires a number of strong mooring lines. These lines must not only be able to withstand the forces generated by the surface forces, but also must be capable of being put in tension to hold the vessel over the site. For well sites in relatively deep water, on the order of 250 feet, the length and required strength of the mooring lines makes mooring of surface production vessels expensive and otherwise unattractive. Additionally, separate winching devices must typically be provided and controlled to tension the mooring lines and hold the vessel over the site. Furthermore, in severe weather, the vessel may have to be moved to a safer location.
Partially submerged vessels such as those referred to as semi-submersibles and described in Corgnet, U.S. Pat. No. 3,961,490 have somewhat reduced the effects of surface forces on production vessels. However, stabilizing the semi-submersible over a discrete location on the sea floor still requires mooring or means for dynamically maintaining the position of the vessel such as by thrusters or the like. If mooring is required, heavy mooring lines must be provided and must be maintained in sufficient tension by winching mechanisms to resist movement of the semi-submersible. Due to the surface forces, the mooring lines must be made strong not only to withstand tensioning, but also the aforesaid surface forces. In relatively deep well sites, the use of mooring lines from an economical point of view becomes unattractive. Again, severe weather may require leaving the site. Accordingly, there is a need to provide a production facility which is not subject to the surface forces and which can be quickly and easily moored over the well site.
Another drawback of prior production facilities relates to the ocean floor valve manifolds which collect and combine the oil or gas from a number of wells to a common riser extending upward to the facility. The manifolding, commonly referred to as a "christmas tree", is typically located in a closed shell at the ocean floor. To service the christmas tree, submarine-type vessels are required to travel from the surface to the shell to transport servicing crews. Submarines are expensive and, especially in rough seas, must be stored on board the vessel when not in use. There is a need, therefore, to provide a means for simple access to a ocean floor mounted shell.